The present invention relates generally to optical devices and, more particularly, to a method, apparatus and system for testing one or more waveguides of an optical planar lightwave circuit device to determine any effect a waveguide may have on the polarization state of light launched into it.
Planar Lightwave Circuit (PLC) devices often have unintended effects on the polarization states of the light propagating through them. Understanding and quantifying these effects in a real-time manner enables better designs of PLC devices to be achieved. Polarization Extinction Ratio (PER) is a measure of the ratio of the electric field amplitude of one polarization state to the electric field amplitude of the other polarization state (e.g., the ratio between the horizontal and vertical polarization components where the components are orthogonal to each other). With respect to PLC devices, the polarization state is typically thought of in terms of a state that is parallel to the substrate of the PLC device, which is typically referred to as the horizontal component, and a state that is perpendicular to the substrate, which is typically referred to as the vertical component. In this case, the PER is the ratio between the horizontal polarization component and the vertical polarization component.
As stated above, a PLC device can have an unexpected effect on the polarization states of the signals propagating through the device. Some of these effects include, for example, scrambling of the polarization states and shifting of the center wavelength within the PLC device, which are both undesirable effects. In the case of an optical demultiplexer PLC, for example, slight shifting of the center wavelength on each channel has been observed.
A known technique for testing a PLC device to determine its effects on the polarization state of light passing through them involves: 1) launching light from a polarization controller into the input of the waveguide of PLC device via a length of optical fiber connected to the polarization controller; 2) receiving the light output from the output of the waveguide of the PLC; and 3) measuring the effects caused by the PLC on the polarization state of the light output from the waveguide. This technique assumes that the polarization state of the light being launched from the end of the optical fiber into the input of the waveguide is the same as the polarization state of the light being output from the polarization controller. In other words, this technique assumes that the optical fiber does not affect the polarization state of the light between the point where the light leaves the polarization controller and the time when the light is launched from the end of the optical fiber into the waveguide input.
U.S. Pat. No. 5,371,597 (hereinafter the xe2x80x9c""597 patentxe2x80x9d) discloses a system and method for using a deterministic method for computing polarization dependent loss. The technique requires four unique input states of polarization in order to compute the polarization dependent loss of an optical device under test (DUT). This patent assumes that each of the four states of polarization, once selected, will be maintained over the length of optical tap, i.e., from the end of the optical tap connected to the polarization controller to the input of the DUT. Therefore, changes in polarization state caused by the optical tap itself are not taken into consideration.
In accordance with the present invention, it has been determined that the state of polarization of light can change between the polarization controller and the end of the optical fiber from which light is launched into the PLC DUT due to a number of variables such as, for example, the type of optical fiber used, the length of the optical fiber used and/or spatial movement of the optical fiber, etc. All of these factors can contribute to the uncertainty of the polarization state of the light at the end of the optical fiber from which light is launched into the PLC device.
Accordingly, a need exists for a method and apparatus that enable the polarization state of light being launched from the end of the optical fiber into the PLC DUT to be measured so that the polarization state of the light launched from the optical fiber end into the PLC can be adjusted, if necessary, to have the proper, or desired, polarization state for testing the DUT. A need also exists for such a device that will enable the polarization state (e.g., the PER and/or PDL) of light output from the waveguide to be determined in order to determine any effects the waveguide had on the polarization state of light that propagateed through it.
The present invention provides a measurement system that is capable of analyzing light at the input of an optical waveguide of an optical device under test (DUT) and/or at the output of the waveguide, preferably at both. At the input of the waveguide, light having a particular polarization state generated by a polarization controller is output from the polarization controller into an end of an optical fiber. The measurement system analyzes the polarization state of the light being launched from the opposite end of the optical fiber to be into the waveguide input of the DUT to determine whether and by how much the polarization state of the light has been changed by the optical fiber. The polarization controller is altered, if necessary, to compensate for any changes in the polarization state caused by the optical fiber. At the output of the waveguide, the measurement system analyzes the polarization state of the light output from the waveguide to determine any effect the waveguide of the DUT had on the polarization state of the light that propagated through it.
In accordance with the preferred embodiment, at the end of the optical fiber furthest from the polarization controller, light being output from the end of the optical fiber is captured and analyzed by the measurement system. In particular, a computer of the measurement system executes one or more algorithms that determine the polarization extinction ration (PER) and/or the polarization dependent loss (PDL) associated with the light output from the end of the optical fiber and determines, based on one or both of these measurements, whether or not the polarization controller needs to be adjusted to compensate for changes in the polarization state of light caused by the optical fiber. If a determination is made that the polarization controller needs to be adjusted, feedback signals generated by the computer are provided to the polarization controller to cause it to adjust the polarization state of the light it is generating to compensate for changes in the polarization state caused by the optical fiber.
At the output of the waveguide, light being output from the waveguide is captured and analyzed by a measurement system. In particular, a computer of the measurement system executes one or more algorithms that analyze the polarization state of the light exiting the waveguide and determine any effect the waveguide of the DUT had on the polarization state of the light launched into the waveguide. This determination may be used for the purpose of, for example, determining whether the waveguide of the DUT is operating properly. As in the case of light output from the distal end of the optical fiber, in this case the PER and/or PDL may be determined and analyzed to determine any effect the waveguide may have had on the polarization state of light that propagated through it.
The measurement system of the present invention that is used to analyze the light being output from the distal end of the optical fiber comprises a polarization controller, a lens, a beam splitter, first and second optical sensors, and processing logic. The polarization controller generates a beam of light having a particular polarization state. The beam of light is coupled into the proximal end of the optical fiber. The lens receives the light output from the distal end of the optical fiber and focuses the light on a beam splitter, which separates the beam of light into first and second polarization components. The polarization components may be orthogonal or non-orthogonal to each other (i.e., they may be 90xc2x0 apart or have some other angle between them, or the light may be circularly polarized). The first optical sensor is positioned to receive the first polarization component from the beam splitter and to convert the light into corresponding electrical signals. The second optical sensor is positioned to receive the second polarization component from the beam splitter and to convert the light into corresponding electrical signals.
The processing logic, which includes the computer and other circuitry, receives the electrical signals from the first and second sensors and processes the electrical signals in a particular manner to make the electrical signals suitable for processing by the computer. The computer processes the electrical signals in accordance with a measurement algorithm. The processing logic determines the polarization state of the light output from the distal end of the otical fiber and, if necessary, provides a compensation feedback signal to the polarization controller to cause it to adjust the polarization state of the light it is launching into the proximal end of the optical fiber. This enables the polarization state of the light being launched from the distal end of the optical fiber into the input of the waveguide to be known and to be controlled.
For light being output from the output of the waveguide, the measurement system comprises the same elements as the measurement system described above, except for the polarization controller. The computer of the processing logic of this measurement system analyzes the light to determine whether the waveguide of the DUT has modified the polarization state, and if so, to what degree. From this analysis, a determination can be made as to any effect that the waveguide had on the polarization state of the light that propagated through it. In this case, the computer also determines the PER and/or the PDL and uses these determinations to determine any effect the waveguide had on the polarization state of the light that propagated through it.
The reason for measuring the ratio between the polarization components (e.g., the PER) is for the purpose of normalizing the data being analyzed. This allows a determination to be made as to the effect that the waveguide has on the polarization state of the light passing through it by comparing the polarization ratio at the input of the waveguide to the polarization ratio of the light at the output of the waveguide, without having to take into account any coupling loss attributable to the DUT.
It is not necessary that the measurement systems that analyze the light at the distal end of the optical fiber and at the output of the waveguide be exclusive of one another. One or more components of each of the systems may be shared amongst the systems. In fact, the combination of the two measurement systems could be viewed as a single measurement system that shares components where possible or desirable, such as, for example, the computer that executes the measurement algorithms to make the aforementioned determinations. However, for ease of illustration and discussion, the present invention will be described as if two totally separate measurement systems are used.